The present invention relates generally to multiplex data communications systems and, more particularly, to a slave circuit for use on a motor vehicle multiplex communications bus to provide remote data input and output for one or more standard nodes connected to the bus.
The use of multiplex wiring systems in motor vehicles have been known for some time to reduce the large number of wires which are used within vehicles. In addition, such multiplex wiring systems benefit the design, assembly and diagnostics of motor vehicles. To promote uniformity and facilitate use of multiplex wiring systems within motor vehicles, the Society of Automotive Engineers (SAE) has adopted a protocol known as the J1850 protocol.
The J1850 protocol is primarily concerned with bus signals within a speed range of from approximately 1 to 100 kilobits-per-second which signals are commonly referred to as Class B signals. Class B signals are required for example for information sharing among systems concerned with control, instrumentation and power train components of a motor vehicle. The J1850 protocol must also be capable of handling bus signals within a speed range of from 10 bits-per-second to 1 kilobits-per-second which signals are commonly referred to as Class A signals. Simple body control applications such as lights including turning and braking indications, dashboard displays and power options such as seat motors and trunk opener solenoids are typical of Class A signals. While Class B signals represent more critical information, Class A signals are important from a wire reduction standpoint because Class A wiring typically accounts for 60 to 65% of the total wiring within a motor vehicle.
Since network nodes capable of handling Class B signals are relatively expensive when compared to devices which are capable of handling the much slower Class A signals, it is undesirable to use Class B nodes for performing Class A operations in most situations. To economically permit the performance of Class A operations from Class B nodes, there is a need for a low cost Class A interface designed for simple input and output functions on a Class B network using the J1850 protocol. Preferably, the interface would be a stand-alone unit which would operate with one or more nodes connected to a Class B network.